The invention concerns an NMR (nuclear magnetic resonance) apparatus comprising a magnet system that is arranged in a cryostat, the cryostat having at least one nitrogen tank for receiving liquid nitrogen, and a room temperature bore for receiving an NMR probehead with a transmitting and receiving system, wherein part(s) of the probehead or the overall probehead can be cooled to cryogenic temperatures by supplying liquid nitrogen via a supply line.
An NMR apparatus of this type, comprising a cryostat and a cooled NMR probehead is disclosed e.g. in U.S. Pat. No. 5,247,256 (document [1]).
FIG. 2 schematically shows the prior art described in [1].
A cryostat 1 contains an inner tank 3a with gaseous helium 4a and liquid helium 5a for cooling a superconducting magnet coil 2, as well as a further tank 3b with gaseous nitrogen 4b and liquid nitrogen 5b, and also an interposed cold shield 27. The nitrogen tank 3b and cold shield 27 are used for thermal shielding of the helium tank 3a in order to minimize thermal losses. The helium tank 3a and the nitrogen tank 3b are thermally insulated with respect to one another and also with respect to the surroundings by means of a vacuum chamber 13.
A room temperature bore 7 for receiving an NMR probehead 8 is disposed in the cryostat 1. A transmitting and receiving system, which generally consists of an RF part 9 and an optional preamplifier 10, is located inside the NMR probehead 8. The RF part 9 typically consists of components such as resonator coils, gradient coils and an RF network. The RF part 9 and the preamplifier 10 are coupled to respective heat exchangers 11, 12, which are connected to the supply line 14.
Liquid nitrogen is removed from an external nitrogen tank 18 for cooling the transmitting and receiving system 9, 10, is guided via the supply line 14, which is connected to the NMR probehead 8 via a separable connection 19, through the heat exchangers 11 and 12, and is subsequently discharged from the probehead 8 to the surroundings. The overall transmitting and receiving system 9, 10 may thereby be cooled to cryogenic temperatures. It is also possible to only cool parts of the transmitting and receiving system 9, 10 to cryogenic temperatures, e.g. by only cooling the RF network or only cooling the coil.
Separate refilling containers are normally used for refilling the nitrogen tank 3b in the cryostat 1 and the external tank 18.
In the simplest form, the liquid nitrogen for cooling the NMR probehead is supplied through an excess pressure in the gas compartment of the external nitrogen tank [2].
The use of pumps for supplying cryogenic fluids [3],[4] was also examined in the past. It is moreover possible to build up a pressure in the external tank by increasing its own pressure, which can be additionally supported by a heating device.
A cryo-coldhead may be provided in the cryostat for liquefying nitrogen or helium [5].
A cryo-coldhead may also be located in the external cryogen tank for producing liquid nitrogen [6].
The waste gas may be returned along the supply line either coaxially with respect to the supply line or through a separate return line [1].
A conventional system including external cryogen tank and supply lines between external tank and NMR probehead requires a relatively large amount of space. In particular, the external cryogen tank occupies valuable space outside of the magnetic field. But space is rare in the laboratories, the sizes of which become increasingly smaller. Both research institutes and industry demand systems that are more compact.
The external cryogen tank causes considerable initial acquisition costs in addition to the cryostat that is expensive per se, and subsequently causes continuous additional maintenance and operating costs. Both the external tank and the inner tank in the cryostat have to be refilled, maintained and supervised, and must have the same or at least comparable safety devices. Mutual adjustment of the filling cycles of both containers is also required. In most cases, the external tank and the nitrogen tank of the cryostat must be refilled at different times, which results in frequent interruptions of the measurement operation and a prolonged downtime of the system. Parallel filling of both tanks is only possible with two refilling containers, which causes additional costs and requires additional space in the laboratory.
Continuous operation is only possible at considerable expense, e.g. by providing a cryo-coldhead on the cryostat PLUS an additional cryo-coldhead on the external cryogen tank. For this reason, at least two additional cryo-cooling machines are required. Compressors and cooling devices are moreover also required.
In contrast thereto, it is the underlying purpose of the present invention to improve an NMR apparatus with a probehead that is cooled to cryogenic temperatures of the above-mentioned type with as simple as possible technical means in such a fashion that the overall apparatus is more compact and requires less space, the operating comfort of the apparatus is increased, and the costs for acquisition, operation and maintenance are clearly reduced compared to conventional comparable devices.